Flextensional transducers were first developed in the 1920s and have been used as underwater transducers since the 1950s. They consist of an active piezoelectric or magnetostrictive drive element and a mechanical shell structure. The shell is used as a mechanical transformer which transforms the high impedance, small extensional motion of the ceramic into low impedance, large flexural motion of the shell. According to the shape of the shell, flextensional transducers are divided into five classes. Flextensional transducers range in size from several centimeters to several meters in length and can weigh up to hundreds of kilograms. They are commonly used in the frequency range of 300 to 3000 Hz. Such transducers can operate at high hydrostatic pressures, and have wide bandwidths with high power output.
A new type of high performance flextensional transducer called the "moonie," was developed by an inventor hereof, and is shown in U.S. Pat. No. 4,999,819. The moonie acoustic transducer is of sandwich construction and is particularly useful for the transformation of hydrostatic pressures to electrical signals. A pair of metal plates are positioned to sandwich a piezoelectric element, with each plate having a cavity formed adjacent to the piezoelectric element. The plates are bonded to the piezoelectric element to provide a unitary structure. The cavities provide a stress-transforming capability which amplifies an incoming compressive stress and converts it to a radial extensional stress in the ceramic.
U.S. Pat. No. 5,276,657, assigned to the same assignee of this application, describes a ceramic actuator that includes a piezoelectric or electrostrictive ceramic disk with conductive electrodes bonded to its major surfaces. A pair of metal end caps include rims that are bonded to ceramic conductive electrodes, respectively. Each end cap is comprised of a solid metal disk having a cavity formed in one surface.
If the ceramic disk is a piezoelectric material, it is poled, during manufacture, across its thickness dimension. If it exhibits an electrostrictive effect or a field-induced antiferroelectric-to-ferroelectric transformation, then it need not be poled. When a potential is applied across the electrodes, the ceramic disk expands in the thickness dimension. At the same time, the ceramic disk contracts in the x and y dimensions causing the end caps to bow outwardly, amplifying the actuation distance created by the contraction of the ceramic disk.
To improve the displacements achievable through actuation of the ceramic disk, an inventor hereof (in U.S. Pat. No. 5,729,077) utilized sheet metal caps (convex outward) joined to opposed planar surfaces of the ceramic substrate. In a sensor embodiment, the sheet metal caps were subjected to a displacement by an instrumentality (i.e., pressure), and a resulting change in voltage across the ceramic substrate was sensed. Due to the shape of the sheet metal caps, the transducer was dubbed a "cymbal" transducer.
Both the moonie and cymbal transducers use a piezoelectric disk (poled in the thickness direction) sandwiched between two metal end caps. The caps contain a shallow cavity on their inner surface. The presence of the cavities allows the caps to convert and amplify the small radial displacement of the disk into a much larger axial displacement normal to the surface of the caps, which contributes to a much larger acoustic pressure output than would occur in the uncapped ceramic.
The cymbal transducer shown in U.S. Pat. No. 5,729,077 is intended for shallow water use when employed as a hydrostatic sensor. If the applied hydrostatic pressure exceeds a certain threshold, the bounding metal caps will deform and collapse, destroying the pressure amplification effect. Accordingly there is need for a cymbal-type transducer that will operate at high depths, without collapsing.